The present invention is directed to a process for the preparation of iminodiacetonitrile (IDAN), a valuable intermediate used to prepare compounds such a iminodiacetic acid.
It is well known that IDAN can be obtained by reacting hexamethylenetetramine (HMTA) or its precursors (ammonia and formaldehyde) with HCN. Eschweiler, Ann., 278, 229-239 (1894); Dubsky et al., Ber., 54, 2659 (1921).
In 1957, Miller U.S. Patent No. 2,794,044 disclosed that IDAN could be prepared in a batch reaction having a yield of about 65% by reacting three moles of HCN, two moles of ammonia, and two moles of formaldehyde in an aqueous solution having a pH of 5.5-6.5. Subsequently, in 1965, Saunders U.S. Pat. No. 3,167,580 taught that IDAN could be produced from the same three reactants in a continuous process carried out at a pH of 7-9 if different mole ratios were used.
It is also known that IDAN can be prepared by the reaction of HMTA and HCN in an acidic aqueous medium. In a batch operation, this reaction can be represented as follows: ##STR1## It can be seen that this reaction only succeeds in converting three of the four nitrogens in the HMTA to the desired product, with the remaining nitrogen forming ammonium by-products. The formation of these by-products requires that the acidic pH of the reaction must be maintained by the addition of acid to the reaction mixture. The actual product is thus a slurry of IDAN crystals in a liquor containing dissolved IDAN, HCN, and other organic and ammonium by-products. These by-products either cannot be separated (which means the liquor cannot be recycled and requires costly effluent treatment) or can only be separated with difficulty. An example is Stutts U.S. Pat. No. 3,412,137 which teaches a batch process for preparing IDAN by reacting an aqueous solution of HMTA with HCN in an aqueous medium buffered with a weak acid (preferably acetic or phosphoric acid) to a pH of 5.5-6.5 at temperatures of 0.degree.-75.degree. C. Cullen U.S. Pat. No. 3,993,681 teaches a batch process wherein HMTA and HCN are reacted in an aqueous solution in the presence of a strong acid at pH 5.5-6.5 and temperatures of 30.degree.-70.degree. C. while maintaining an excess of HCN in the reaction mix. Koenig Canadian Patent No. 684,850 prepares IDAN by reacting HMTA (or its precursors) with HCN in aqueous solution and then acidifying with a strong mineral acid. Similarly, the recent Suchland et al. U.S. Pat. No. 4,307,037 reacts HMTA with HCN in an aqueous medium initially having a pH of 5.5-7.5 and then further acidifying the reaction mixture by 0.5-3.5 pH units by the addition of an acid (preferability sulfuric) at a temperature of 30.degree.-90.degree. C. All of the foregoing references teach batch operations, use the foregoing mole ratio of six moles of HCN to one mole of HMTA, are conducted in an aqueous medium whose acidic pH is maintained by the addition of acid, and are conducted at fairly low temperatures. In contrast, Philbrook et al. U.S. Pat. No. 3,886,198 discloses a continuous process for the preparation of IDAN by passing a mixture of HMTA, HCN, and a strong acid through a tubular reactor. This continuous process differs from the batch process in that it involves five to seven moles of HCN per mole of HMTA and must be operated at a higher temperature, which is an additional disadvantage.
An alternate route to IDAN involves the use of glycolonitrile as a reactant. This is disclosed in Gaudette et al. U.S. Pat. No. 3,904,668 wherein an aqueous mixture of HMTA, HCN, and glycolonitrile is continuously passed through a tubular reactor. The process has the advantage of converting all four nitrogens in the HMTA to the IDAN product.
A more recently developed route involves the addition of formaldehyde, rather than an acid, to maintain the pH of the reaction mixture. A continuous process of this type for the formation of IDAN by the reaction of HMTA, HCN, and formaldehyde is shown in Gaudette et al. U.S. Pat. No. 3,988,360. Like the Gaudette '668 patent, the process theoretically converts all four nitrogens in the HMTA to IDAN, with no formation of ammonium by-product: EQU (CH.sub.2).sub.6 N.sub.4 +8HCN+2HCHO.fwdarw.4HN(CH.sub.2 CN).sub.2 +2H.sub.2 O
However, because it must be operated at higher temperatures, usually 130.degree.-150.degree. C., this continuous process has been found to suffer from low yields caused by the formation of by-products. It has now been found that batch operation of the reaction described in the Gaudette '360 patent results in a significantly higher yield of a pure, colorless IDAN. It is believed that the reason for this result is the lower temperature at which this batch process may be operated. In contrast, the higher temperature of a continuous process such as that described in the Gaudette '360 patent favors the occurrence of side-reactions which result in the formation of undesirable by-products and color bodies.
In short, the present invention's use of formaldehyde instead of acid as well as the use of lower temperatures both tend to minimize the formation of undesirable by-products and give a higher yield of IDAN.